NUMBERS : DYNAMICS 69 



in all their aspects) and synecology (the study of 

 animal communities). It is clear that the study of 

 the autecology of the numbers of any species involves 

 inevitably a consideration of the synecology of the 

 community in which it lives. It is for this reason 

 that full ecological surveys are of such importance in 

 ecology. It has already been shown how ecological 

 surveys serve to define the animal inter-relations 

 existmg in a habitat. It is not true that a dis- 

 turbance of the numbers of one species affects all 

 the several hundred other members of the community 

 in which it lives. It is true that we cannot easily 

 predict that it will not indirectly affect some other 

 species, apparently unconnected with it. We might 

 easily find that some insect in town gardens was 

 affected by the decrease in sparrows resulting from 

 the decrease in horse-drawn vehicles and consequent 

 scarcity of food. It has been suggested with some 

 evidence that the decrease in summer diarrhoea in 

 children in London may be due to the same cause — 

 involving the decrease of horse manure in which 

 breed house-flies that carry the germs of this disease 

 (Graham Smith, 1930). 



There is a fourth point of importance in connexion 

 with the Hmiting factors to increase in animals. 

 Factors which act as checks may be divided into 

 two classes. Those which act irrespective of the 

 density of the population. In this class come the 

 effects of climate, of general food shortage not due 

 to crowding, in fact, all external catastrophes. There 

 is a second class of factors which depend upon the 

 density of the population. An obvious example is 

 disease due to a parasite and caused by overcrowding. 

 And there are the factors (still mostly obscure) caus- 

 ing decreased viability or reproductive rates at very 

 low densities (see Allee, 1931 and Chapter V). The 

 distinction between these two modes of population 

 control is very important. One acts in a random 

 manner whether animals are abundant or scarce. 



